What is a hotspot and how is it measured?

Mercury hotspots can occur in several forms. Locations that receive high mercury loading are referred to as “deposition hotspots”. Areas where mercury concentrations are elevated in fish and wildlife are known as “biological hotspots”. Scien-tists compiled mercury in fish, common loons, bald eagles, mink and river otter and generated a preliminary map of nine biological hotspots in freshwater ecosystems. Except for two locations, these biological hotspots are not necessarily linked to any one particular source and are therefore areas that scientists believe are likely associated with airborne mercury emissions. The two exceptions are the biological hotspots near large point sources in southeastern New Hampshire (#3) and a defunct chlorine factory in Orrington, Maine (#6).

The preliminary map of biological hotspots for freshwater ecosystems shown in Figure 13 represents nine areas that meet the following criteria.

1. Two or more organisms with mercury levels consistently above thresholds for documented adverse effects.
2. A relatively large area impacted.
3. A high density of measurements showing elevated mercury in biota.
4. A substantial deviation in mercury levels from the surrounding landscape.

The threshold level used for identifying potential fish hotspots is 0.16 ppm (wet weight, whole body mercury levels). Scientists have determined that this level potentially poses a population level risk for fish-eating birds such as the common loon (Evers et al. 2004). For loons, an area is highlighted as a possible location of concern if values for adult blood levels exceed 3.0 ppm or egg levels greater than 1.3 ppm. This level has been identified as a threshold for ecological effects on the physiology, behavior, reproduction, and survival of common loons (see Table 1). In bald eagle young an estimated threshold of 0.7 ppm of mercury (wet weight) in blood is used. This estimate corresponds to the mercury level documented in bald eagles at locations where common loon mercury exceeds 3.0 ppm. Mink and otter are known to exhibit sublethal toxicity at fur mercury levels above 20 ppm.

Where are the hotspots in the region?

At the present time, nine major biological hotspots have been identified in the area from New York state to Nova Scotia. Seven of these biological hotspots are not associated with a known point source. The map is a preliminary depiction of the extent of biological hotspots as it is possible that more biological hotspots will emerge as additional information is collected and as areas of concern are potentially identified in forest ecosystems.

Fish and loons are most broadly represented on the map, consistent with the large databases for these organisms. The hotspots for wildlife are evenly distributed, except for the bald eagle which occurs only in Maine. It is also noteworthy that six of the nine biological hotspots show elevated mercury in three or more organisms.

Why are some areas “hot”?

Many reasons exist for the occurrence of biological hotspots. Beyond long distance transport of mercury emissions, the reasons include hydrological impacts on reservoirs (such as in western Maine), local emission sources (like the defunct chlorine plant in Maine) and lakes with chemical conditions that are conducive to methylmercury production. This latter category encompasses much of the region and includes acidic lakes in the Adirondack Mountains of New York, Quebec and Nova Scotia.

Kejimkujik National Park in Nova Scotia provides an interesting case study in the patterns of biological hotspots. Ninety-two percent of adult loons at Kejimkujik had blood mercury levels >4.0 ppm, and several individuals had among the highest levels found in breeding common loons across North America (up to 7.8 ppm). Reduced reproduction has been observed in loons at Kejimkujik for the last 15 years (Kerekes et al. 1994 and Burgess et al. 1998). Yet, the map of mercury deposition (Figure 4) indicates that Kejimkujik receives relatively low mercury deposition compared with much of the region. Acidic surface waters in Kejimkujik have the ideal conditions for converting mercury to methylmercury, which facilitates the uptake and accumulation of mercury in the food chain. The Kejimkujik hotspot underscores the importance of achieving significant reductions in mercury deposition across the landscape, as well as the importance of reducing acidity in surface waters in order to achieve biological recovery.